1. Field of Art
This invention relates to mode locked surface acoustic wave oscillators used as comb spectrum generators, and more particularly to stabilization thereof by means of the pulse repetition frequency.
2. Description of the Prior Art
There are a variety of electronic technologies, such as frequency synthesizers employing electronic selection of various combinations of reference frequencies, which require sets of stable reference signal sources. One such device which has found favor is a comb spectrum generator employing a surface acoustic wave delay line as the frequency determining element in an oscillator. As is known, if the oscillatory loop includes a nonlinear device to suppress low amplitude signals but sustain higher amplitude signals, such as a repressed amplifier or a diode signal expander, the oscillations can be maintained in a repetitive pulse mode, rather than settling into a continuous wave mode. The pulses each include frequency components which are within the bandwidth of the delay line and related thereto by integer numbers; similarly, the repetition frequency of the pulses are related to the inverse of the time delay of the SAW device in an integer fashion. The use of surface acoustic wave devices provides the inherent advantages of small device size and reproducible manufacturing inherent in microelectronic processing technology. Further, the surface acoustic wave devices avoid the need for post-manufacture trimming to precise characteristics, as may be the case in bulk mode crystal devices.
The surface acoustic wave delay line should have a reasonable insertion loss (on the order of 20 dB, or less) while at the same time having a sufficient broad band of frequency response to provide the desired comb spectrum in the output. Depending upon the use to which a device is to be put, the required stability may be as great as one tenth of a part per million. This stability is of course dependent upon the range of temperatures of the intended environment of the device and the sensitivity of the device to temperature variations. For instance, a surface acoustic wave device utilizing a quartz substrate may have adequate temperature stability, but since the piezoelectric effect is not very strong, many fingers would be required in the piezoelectric coupling transducers; since the bandwidth is inversely related to the number of elements in the transducers, selection of a quartz substrate for temperature stability would be at the expense of wide bandwidth. On the other hand, lithium niobate substrates have a sufficiently strong piezoelectric effect to permit coupling signals to and from the SAW device with transducers having only a few fingers, while maintaining relatively low insertion loss, and thereby achieving a broad band frequency response. But, the lithium niobate substrate has a temperature sensitivity which is higher than that of quartz, and in some applications is excessive. Therefore, the desired characteristics of low insertion loss, wide bandwidth and frequency stability cannot be provided in a surface acoustic wave comb spectrum generator without compensation, or control over the temperature of the environment.
In the past, (eg, see: M. Gilden, "Stabilized SAW Comb Spectrum Generator," 1977 Ultrasonics Symposium, p. 927), injection phase locking for frequency stabilization of the comb spectrum generator was achieved by injecting a low level RF CW signal into the feedback loop. However, stabilization was achievable only over an inadequately small temperature range. A similar stability method utilized subharmonic injection phase locking at the pulse repetition frequency. However, because the energy in the subharmonic synchronizing pulses is concentrated at the low end of the frequency spectrum, sufficient interaction with the RF components of the comb signal for adequate stabilizing effect is obtainable only for excessively large amplitudes of subharmonic injection signals, with attendant large background noise. The presence of noise in the output rendered a comb spectrum generator inadequate for applications of the type requiring the stability which was sought.